Dynamically matched microwave antenna for tissue ablation

ABSTRACT

A microwave ablation probe for providing microwave energy to tissue is disclosed. The probe includes a feedline having an inner conductor, a secondary inner conductor, an insulating spacer, and an outer conductor. The inner conductor is slidably disposed within the secondary inner conductor. The feedline also includes a radiating portion having an extruded portion of the inner conductor centrally disposed therein, wherein longitudinal movement of the inner conductor relative to the feedline tunes the radiating portion.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.12/265,024 filed on Nov. 5, 2008, now U.S. Pat. No. 8,280,525, whichclaims the benefit of and priority to U.S. Provisional Application Ser.No. 60/988,699 filed on Nov. 16, 2007, the entirety of each of which isincorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave applicator probesused in tissue ablation procedures. More particularly, the presentdisclosure is directed to a microwave probe that can be tuned duringablation procedures to obtain a desired impedance match.

2. Background of Related Art

Treatment of certain diseases requires destruction of malignant tissuegrowths (e.g., tumors). It is known that tumor cells denature atelevated temperatures that are slightly lower than temperaturesinjurious to surrounding healthy cells. Therefore, known treatmentmethods, such as hyperthermia therapy, heat tumor cells to temperaturesabove 41° C., while maintaining adjacent healthy cells at lowertemperatures to avoid irreversible cell damage. Such methods involveapplying electromagnetic radiation to heat tissue and include ablationand coagulation of tissue. In particular, microwave energy is used tocoagulate and/or ablate tissue to denature or kill the cancerous cells.

Microwave energy is applied via microwave ablation antenna probes whichpenetrate tissue to reach tumors. There are several types of microwaveprobes, such as monopole, dipole, and helical. In monopole and dipoleprobes, microwave energy radiates perpendicularly from the axis of theconductor. Monopole probe (e.g., antenna) includes a single, elongatedmicrowave conductor surrounded by a dielectric sleeve, having aconductor exposed at the end of the probe. Dipole probes have a coaxialconstruction including an inner conductor and an outer conductorseparated by a dielectric portion. More specifically, dipole microwaveantennas have a long, thin inner conductor which extends along alongitudinal axis of the probe and is surrounded by an outer conductor.In certain variations, a portion or portions of the outer conductor maybe selectively removed to provide for more effective outward radiationof energy. This type of microwave probe construction is typicallyreferred to as a “leaky waveguide” or “leaky coaxial” antenna.

In helical probes, microwave energy is directed in a forward direction.This is due to microwave energy radiating perpendicularly from theantenna, which when in helical configuration directs the energy waves ina forward direction. In helical probes the inner conductor is formed ina uniform spiral pattern (e.g., a helix) to provide the requiredconfiguration for effective radiation.

Conventional microwave probes have a narrow operational bandwidth, awavelength range at which optimal operational efficiency is achieved,and hence, are incapable of maintaining a predetermined impedance matchbetween the microwave delivery system (e.g., generator, cable, etc.) andthe tissue surrounding the microwave probe. More specifically, asmicrowave energy is applied to tissue, the dielectric constant of thetissue immediately surrounding the microwave probe decreases as thetissue is cooked. The drop causes the wavelength of the microwave energybeing applied to tissue to increase beyond the bandwidth of the probe.As a result, there is a mismatch between the bandwidth of conventionalmicrowave probe and the microwave energy being applied. Thus, narrowband microwave probes may detune as a result of steam generation andphase transformation of the tissue hindering effective energy deliveryand dispersion.

SUMMARY

The present disclosure provides for a microwave ablation probe which canbe dynamically matched and/or tuned during ablation. As tissue isablated, the radiating portion of the probe is actively tuned so that anoptimal impedance match is achieved for a desired procedure. This isaccomplished by adjusting the shape, size and/or dielectric propertiesof the components of the probe (e.g., adjusting the length of theconductors, insulating layers, and the like). In monopole and/or dipoleantennas, the length of an inner conductor is adjusted to create a moreefficient radiator. In dipole antennas, the length of the outer andinner conductors is adjusted such that a predetermined wavelengthdistance at the radiating portion is maintained despite frequencychanges (e.g., inner and outer conductors being ¼ wavelength long tomaintain balanced behavior of a ¼ wavelength dipole). In anotherembodiment, dielectric properties of the radiating portion are adjustedby using materials with thermally changing dielectric properties; thus,as the temperature of the tissue and the probe changes during ablationthe dielectric properties of the probe are automatically adjusted.

According to one embodiment of the present disclosure a microwaveablation probe for providing microwave energy to tissue is disclosed.The probe includes a feedline having an inner conductor, a secondaryinner conductor, an insulating spacer, and an outer conductor. The innerconductor is slidably disposed within the secondary inner conductor. Thefeedline also includes a radiating portion having an extruded portion ofthe inner conductor centrally disposed therein, wherein longitudinalmovement of the inner conductor relative to the feedline tunes theradiating portion.

According to another embodiment of the present disclosure a microwaveablation probe for providing microwave energy to tissue is disclosed.The probe includes a feedline having an inner conductor, an insulatingspacer and an outer conductor, and a radiating portion having anextruded portion of the inner conductor which is centrally disposedtherein. The probe also includes a choke disposed around at least aportion of the feedline and configured to confine the microwave energyto the radiating portion. The choke includes a conductive housing havinga chamber for storing a cooling dielectric liquid.

According to a further embodiment of the present disclosure a microwaveablation probe for providing microwave energy to tissue is disclosed.The probe includes a feedline having an inner conductor, an insulatingspacer and an outer conductor, a radiating portion including a radiatingportion including at least a portion of the inner conductor centrallydisposed therein. The probe also includes one or more loadings having anelectric field-dependent dielectric material, wherein one or more of thedielectric properties of the electric field-dependent dielectricmaterial varies in response to the electric field supplied thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic diagram of a microwave ablation system accordingto the present disclosure;

FIG. 2 is a perspective cross-sectional view of a microwave ablationprobe according to the present disclosure;

FIGS. 3A-C are side cross-sectional views of the microwave ablationprobe of FIG. 2;

FIG. 4 is a perspective cross-sectional view of the microwave ablationprobe having liquid cooled choke according to the present disclosure;and

FIG. 5 is a perspective cross-sectional view of one embodiment of themicrowave ablation probe having a thermally reactive dielectric materialtherein according to the present disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure will be describedherein below with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail.

FIG. 1 shows a microwave ablation system 10 which includes a microwaveablation probe 12 coupled to a microwave generator 14 via a flexiblecoaxial cable 16 that is coupled to a connector 18 of the generator 14.The generator 14 is configured to provide microwave energy at anoperational frequency from about 500 MHz to about 2500 MHz.

During microwave ablation, the probe 12 is inserted into tissue andmicrowave energy is supplied thereto. As tissue surrounding the probe 12is ablated, the tissue undergoes desiccation and denaturization whichresults in a drop of the effective dielectric constant of the tissue.The drop in the effective dielectric constant, in turn, lengthens thewavelength of the microwave energy. Since the frequency is held constantduring ablation, the increase in the wavelength results in the increaseof the operational frequency. At the outset the probe 12 is at aninitial match point—a predetermined operational frequency that increasesto a higher frequency as the ablation continues. Thus, to maintain animpedance match between the probe 12 and the generator 14, the radiatingproperties of the probe 12 are dynamically adjusted throughout theprocedure. This is accomplished by modifying the geometry and/or thedielectric properties of the probe 12.

FIG. 2 shows one embodiment of the probe 12 including a feedline 26, achoke 28 and an adjustable radiating portion 30. The feedline 26 extendsbetween the distal end of the probe 12 where the feedline 26 is coupledto the cable 16, to the radiating portion 30. The feedline 26 isconstructed from a coaxial cable having an inner conductor 20 (e.g.,wire) surrounded by an insulating spacer 22 which is then surrounded byan outer conductor 24 (e.g., cylindrical conducting sheath). In oneembodiment, the feedline 26 may have a diameter of 0.085 inches and theinsulating spacer 22 may have a dielectric constant of 1.7.

The feedline 26 may be flexible or semi-rigid and may be of variablelength from a proximal end of the radiating portion 30 to a distal endof the cable 16 ranging from about 1 to about 10 inches. The innerconductor 20 and the outer conductor 24 may be constructed from avariety of metals and alloys, such as copper, gold, stainless steel, andthe like. Metals may be selected based on a variety of factors, such asconductivity and tensile strength. Thus, although stainless steel haslower conductivity than copper and/or gold, it provides the strengthrequired to puncture tissue and/or skin. In such cases, the inner andouter conductors 20 and 24 may be plated with conductive material (e.g.,copper, gold, etc.) to improve conductivity and/or decrease energy loss.

In one embodiment, the feedline 26 includes a secondary inner conductor23, as shown in FIG. 3A, having a tubular structure which surrounds theinner conductor 20. The inner conductor 20 is slidably disposed withinthe secondary inner conductor 23 (e.g., moves within the secondary innerconductor 23 while maintaining smooth continuous contact therewith),such that the inner conductor 20 can be slid in either the proximaland/or distal direction to tune the inner conductor 20 to a desiredoperational frequency. The inner conductor 20 and the secondary innerconductor 23 are in electromechanical contact, allowing the innerconductor 20 to slide in and out of the feedline 26 during tuning whilecontinuing to conduct microwave energy.

As shown in FIG. 3B, the feedline 26 includes one or more grooves 25which mechanically interface with one or more corresponding stop members27 disposed on the inner conductor 20. The groove 25, may be disposed inthe secondary inner conductor 23 and/or the insulative spacer 22. Thegroove 25 in conjunction with the corresponding stop member 27, guidesand limits the movement of the inner conductor 20 as the inner conductor20 is slid within the feedline 26. Further, the groove 25 and stopmember 27 combination provides for additional conductive contact betweenthe secondary inner conductor 23 and the inner conductor 20. Inembodiments, the location of the groove 25 and the stop member 27 may beinterchanged, such that the groove 25 may be disposed within the innerconductor 20 and the stop member 27 may be disposed on the secondaryinner conductor 23.

With reference to FIG. 2, the choke 28 of the probe 12 is disposedaround the feedline 26 and includes an inner dielectric layer 32 and anouter conductive layer 34. The choke 28 confines the microwave energyfrom the generator 14 to the radiating portion 30 of the probe 12thereby limiting the microwave energy deposition zone length along thefeedline 26. The choke 28 is implemented with a quarter wave short byusing the outer conductive layer 34 around the outer conductor 24 of thefeedline 26 separated by the dielectric layer 32. The choke 28 isshorted to the outer conductor 24 of the feedline 26 at the proximal endof the choke 28 by soldering or other means. In embodiments, the lengthof the choke 28 may be from a quarter to a full wavelength. The choke 28acts as a high impedance to microwave energy conducted down the outsideof the feedline 26 thereby limiting energy deposition to the end of theprobe. In one embodiment, the dielectric layer 32 is formed from afluoropolymer such as tetrafluorethylene, perfluorpropylene, and thelike and has a thickness of 0.005 inches. The outer conductive layer 34may be formed from a so-called “perfect conductor” material such as ahighly conductive metal (e.g., copper).

As shown in FIG. 3C, the choke 28 is configured to slide atop thefeedline 26 along the longitudinal axis defined by the probe 12. Slidingthe choke 28 in either proximal and/or distal direction along thefeedline 26 provides for adjustment of the length of the radiatingportion 30. The choke 28 includes a groove 33 disposed within thedielectric layer 32. The groove 33 is configured to mechanicallyinterface with a stop member 35 that is disposed on the outer conductor24. The stop member 35 guides the sliding of the choke 28 along thelength of the groove 33.

Moving one or both of the inner conductor 20 and the choke 28 relativeto the feedline 26 allows for adjustment of the length of the radiatingportion 30, such as adjusting the choke 28 and the inner conductor 20 tobe ¼ wavelength long as the ablation continues to maintain ½ wavelengthdipole. In embodiments, the inner conductor 20, the feedline 26 and thechoke 28 may have markings and/or indicia thereon to indicate desiredwavelength adjustment positions.

In one embodiment, the grooves 25 and 33 and/or the stop members 27 and35 may include one or more detents (not explicitly shown) which providetactile feedback when the choke 28 and/or inner conductor 20 are slidalong the feedline 26. This allows for more precise movement of thecomponents and tuning of the radiating portion 30.

The probe 12 further includes a tapered end 36 which terminates in a tip38 at the distal end of the radiating portion 30. The tapered end 36allows for insertion of the probe 12 into tissue with minimalresistance. In cases where the radiating portion 12 is inserted into apre-existing opening, the tip 38 may be rounded or flat. The tapered end36 may be formed from any hard material such as metal and/or plastic.

FIG. 4 shows another embodiment of the probe 12 having a liquid-cooledchoke 40 that includes a cylindrical conducting housing 42 having achamber 44 and defining a cylindrical cavity 46 which surrounds thefeedline 26. The housing 42 is formed from a conducting metal such ascopper, stainless steel, and/or alloys thereof. The housing 42 includesone or more inlet tubes 48 and outlet tubes 50 which cycle a coolingdielectric liquid 52 (e.g., water, saline solution, and the like)through the chamber 44. The liquid 52 may be supplied by a pump (notexplicitly shown) configured to adjust the flow rate of the liquid 52through the chamber 44. As the liquid 52 is supplied into the choke 40,the heat generated by the feedline 26 is removed. Further, compoundsused in the liquid 52 may be adjusted to obtain a desired dielectricconstant within the choke 28. This may be useful in multi-frequencyprobes allowing the resonant frequency of the choke 28 to be adjusted byfilling the chamber 44 with varying fluid volume and/or varying theratio of air and liquid therein.

The housing 42 also includes an O-ring 54 having an opening 56 allowingthe O-ring 54 to fit within the chamber 44. As the chamber 44 is filledwith the liquid 52, the liquid 52 pushes the O-ring 54 in the distaldirection within the chamber 44. The O-ring 54 fits the walls of thechamber 44 in a substantially liquid-tight fashion preventing the liquid52 from seeping into a distal portion 58 of the chamber 44. This allowsselective or automatic adjustment of the cooling temperature of thechoke 28 by limiting the volume of the chamber 44 being filled with theliquid 52.

More specifically, the O-ring 54 is formed from rubber, silicone rubberand other elastomer material such that the frictional forces between theO-ring 54 and the housing 42 maintain the O-ring 54 in position untilthe flow rate of the liquid 52 is sufficient to shift the O-ring 54 inthe distal direction. In one embodiment, the distal portion 58 includessloping or chamfered walls 60 inside the chamber 44. As the O-ring 54 ispushed in the distal direction, the sloping walls 60 compress the O-ring54 which requires an increase in the flow rate of the liquid 52. Thisprovides for a counter-force that pushes back against the flow of theliquid 52 requiring an increase in the flow rate if additional fillingof the chamber 44 (e.g., additional cooling of the choke 28) is desired.Once the liquid 52 is withdrawn from the choke 28, the O-ring 54 ismoved back into its original position (e.g., in the proximal direction)by the compression of the walls 60.

FIG. 5 shows a further embodiment of the probe 12 having a ferroelectricmaterial therein. More specifically, the probe 12 includes an internalferroelectric loading 70 at a distal end of the feedline 26 and anexternal ferroelectric loading 74 at the distal end of the innerconductor 20. In one embodiment, the internal ferroelectric loading 70may be have a length corresponding to the quarter wave of the microwavefrequency and act as a dynamic quarter-wave transformer.

The ferroelectric loadings 70 and 74 include ferroelectric material suchas lead zirconate, lead titanate, barium titanate, and the like.Ferroelectric materials provide for dynamic matching of the probe 12 tothe tissue due to changing dielectric properties of such materials whenDC electric field is applied across thereof during application ofmicrowave energy to the probe 12 such that the DC electric field biasesthe ferroelectric material. The DC electric field is supplied to theloadings 70 and 74 through the outer conductor 24 and inner conductor 20respectively. As the DC electric field is supplied to the loadings 70and 74, the dielectric constant thereof is varied. The “+” and “−”illustrate one possible polarity of DC electric field within the probe12. As the wavelength of the frequency of operation increases due todesiccation of the tissue, the DC electric field is supplied to theloadings 70 and 74 is also adjusted accordingly to increase thedielectric constant accordingly. This counteracts the claiming of theprobe 12 due to the changes in the tissue. In one embodiment, the DCelectric field supply (not explicitly shown) may be controlled via afeedback loop by the generator 14 based on impedance measurement of theprobe 12 and the cable 16 and other methods within purview of thoseskilled in the art. In another embodiment, the supply of the DC currentmay be varied in a predetermined fashion over time based on empiricallaboratory measurements.

The described embodiments of the present disclosure are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present disclosure. Various modifications andvariations can be made without departing from the spirit or scope of thedisclosure as set forth in the following claims both literally and inequivalents recognized in law.

What is claimed is:
 1. A microwave ablation probe for providingmicrowave energy to tissue, the microwave ablation probe comprising: afeedline including an inner conductor, a secondary inner conductor, aninsulating spacer, and an outer conductor, the inner conductor beinglongitudinally movable relative to the secondary inner conductor whilemaintaining electro-mechanical contact therewith; a radiating portionincluding at least a portion of the inner conductor centrally disposedwithin the radiating portion; and a choke disposed around at least aportion of the feedline and configured to confine the microwave energyto the radiating portion, the choke including a conductive housinghaving a chamber for storing a cooling dielectric liquid.
 2. Themicrowave ablation probe according to claim 1, wherein the chokeincludes at least one inlet tube and at least one outlet tube configuredto supply the cooling dielectric liquid into the chamber.
 3. Themicrowave ablation probe according to claim 1, wherein the chokeincludes an O-ring slidably disposed within the chamber, such that whenthe cooling dielectric liquid is supplied thereto the O-ring is moved ina distal direction.
 4. The microwave ablation probe according to claim1, wherein the cooling dielectric liquid is selected from the groupconsisting of water and saline solution.
 5. The microwave ablation probeaccording to claim 1, further including a tapered end having a tipdisposed at a distal end of the radiating portion.
 6. A microwaveablation probe for providing microwave energy to tissue, the microwaveablation probe comprising: a feedline including an inner conductor, asecondary inner conductor, an insulating spacer, and an outer conductor,the inner conductor being longitudinally movable relative to thesecondary inner conductor while maintaining electro-mechanical contacttherewith; a radiating portion including at least a portion of the innerconductor centrally disposed within the radiating portion; and at leastone loading including a direct current electric field-dependentdielectric material, wherein at least one dielectric property of thedirect current electric field-dependent dielectric material varies inresponse to the DC electric field supplied thereto.
 7. The microwaveablation probe according to claim 6, further including: a tapered endhaving a tip disposed at a distal end of the radiating portion.
 8. Themicrowave ablation probe according to claim 6, wherein the directcurrent electric field-dependent dielectric material is a ferroelectricmaterial selected from the group consisting of lead zirconate, leadtitanate and barium titanate.
 9. The microwave ablation probe accordingto claim 6, further comprising: an internal loading disposed at a distalend of the feedline; and an external loading disposed at a distal end ofthe inner conductor.
 10. The microwave ablation probe according to claim9, wherein the internal loading is a dynamic quarter-wave transformer.11. The microwave ablation probe according to claim 9, wherein theinternal loading is of a first polarity and the external loading is of asecond polarity.